The complement protein C5 is a central component of the complement system; a key part of the innate immune system. The complement system is an intricate immune survival system with numerous tasks in tightly controlled, diverse processes. One of its functions is as first line host defense against infection by other organisms by discriminating healthy host tissues from cellular debris and apoptotic and necrotic cells. Furthermore, it is involved in clearance of immune complexes, regulation of the adaptive immune response, promotion of tissue regeneration, angiogenesis, mobilization of stem cells and development of the central nervous system (Woodruff et al. Mol Immunol 2011, 48 (14):1631-1642); Ricklin et al. Nat Immunol 2010, 11(9):785-795). Any trigger, for example erroneous or unrestricted activation or insufficient regulation, that disturbs the fine balance of complement activation and regulation may lead to pathologic conditions including self-attack of the host's cells leading to extensive tissue damage.
The complement system consists of about 30 proteins. There are three pathways to initiate complement immunity; the classical pathway that employs C1q to recognize immune complexes on the surface of cells, the lectin pathway that is initiated when mannose-binding lectin (MBL) recognizes certain sugars and the alternative pathway that is initiated spontaneously by hydrolysis of complement factor 3 (C3), a process suppressed by certain mammalian cell surface molecules not present on invading pathogens. The alternative pathway also acts as an amplification loop for the complement system. All three pathways converge at the level of C3. Cleavage of C3 into C3a and C3b leads to the formation of a convertase that in turn cleaves complement factor 5 (C5) into C5a and C5b. C5a is a very potent attractant of various immune cells while C5b oligomerizes with C6-9 to form a pore known as the membrane attack complex (MAC) or sometimes the terminal complement complex (TCC). Activation of the complement system leads to a number of mechanisms with the purpose of neutralizing the pathogen; formation of MAC on the surface of a cell such as an invading bacteria lead to lysis, deposition of C3 and C4 cleavage products C3b and C4b aids opsonization leading to phagocytosis of the pathogen by macrophages and anaphylatoxins such as C3a and C5a attracts monocytes and neutrophils to the site of activation, upregulates surface markers leading to increased immunologic susceptibility and to the release of cytokines.
C5 is a 190-kDa glycoprotein comprised of 2 disulfide-linked polypeptide chains, alpha and beta, with a molecular mass of 115 and 75 kDa, respectively (Tack et al. Biochem 1979, 18:1490-1497). Haviland et al. (J Immun 1991, 146: 362-368) constructed the complete cDNA sequence of human complement pro-C5, which is predicted to encode a 1,676-amino acid pro-molecule that contains an 18-amino acid leader peptide and a 4-amino acid linker separating the beta and alpha chains. Blockade of C5 cleavage into C5a and C5b prevents MAC formation and formation of the pro-inflammatory C5a but leaves the upstream complement effector system intact allowing the C3/C4 mediated opsonization.
The complement system's key role in the defense against pathogens in general makes it an interesting target for pharmaceutical intervention. This is emphasized by the fact that many mutations or impaired regulation of complement is involved in various diseases and conditions. These include increased susceptibility to auto-immune diseases such as systemic lupus erythematosis (SLE) where deposition of immune complexes triggers the classical pathway (Manderson et al. Annu Rev Immunol 2004, 22:431-456). In addition, mutations of the complement proteins C1-C5 often result in SLE or SLE like symptoms. Other autoimmune diseases with a strong involvement of the complement system are rheumatoid arthritis (RA) where immune complexes may activate complement in the RA joint, Sjögren's syndrome, dermatomyositis and other autoantibody driven diseases such as Guillain-Barré syndrome (GBS), Fisher syndrome (Kaida et al. J. Neuroimmun 2010, 223:5-12) different types of vasculitis, systemic sclerosis, anti-glomerular basement membrane (anti-GBM) and anti-phospholipid syndrome (APS) (Chen et al. J Autoimmun 2010, 34:J276-J286).
The complement system is furthermore involved in neurodegenerative disorders such as Alzheimer's disease (AD) where AP plaques directly activate the complement system leading to C5a mediated recruitment of microglia. This was further confirmed when a C5aR antagonist was shown to be neuroprotective in a mouse model of AD (Fonseca et al. J Immunol 2009, 183:1375-1383). Auto-antibodies against the acetylcholine receptor and subsequent complement activation is the most common cause to myasthenia gravis, a disease that affects the neuromuscular junction (Toyka and Gold. Schweizer Archive Neurol Psych 2007, 158:309-321). MAC formation is involved in the pathophysiology of multiple sclerosis (MS) (Oh et al. Immunol Res 2008, 40:224-234). Also in Parkinson's disease, Huntington's disease and prion diseases such as Creutzfeld-Jacob disease, complement activation is a part of the pathology (Bonifati and Kishore, Mol Imnmunol 2007, 44:999-1010). In wound healing, inflammatory responses are a key component to restore tissue homeostasis and the complement system is involved in the early recognition of damaged tissue. However, in models of chronic wounds and severe burns, for example, inhibition of complement by e. g. C1 inhibitor resulted in improved healing and decreased tissue damage suggesting that complement. Furthermore, various complement deficiencies, such as exemplified by the C4 knockout mouse, have been found to be protective against long-term tissue damage resulting from wounds (reviewed in Cazender et al. Clinical and Developmental Immunology 2012, on-line publication). Lately it has been shown that tumor growth and proliferation is facilitated by complement activation, in particular by C5a, and that blockade of the C5a receptor slows down this process. In addition, mice lacking C3 display significantly slower tumor growth than wild-type littermates (Markiewski et al. Nat Immunol 2008, 9:1225-1235).
Dysfunctional complement regulation is the cause of several rare to ultra-rare conditions, such as paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS), where hemolysis is a key feature in the pathology. In PNH, a clone of hematopoetic stem cells with mutated PIG-A gene encoding phosphatidylinositol N-acetylglucosaminyltransferase subunit A take over the pool of blood cells. This mutation leads to loss of GPI anchored proteins such as the complement regulators CD55 and CD59. Red blood cells lacking CD55 and CD59 on the surface are exposed for complement mediated lysis by MAC. Clinically, PNH is manifested by hemolysis leading to anemia, thrombosis and bone marrow failure. Atypical HUS is caused by mutations in regulatory proteins of mainly the alternative pathway, such as by mutations in factor H.
The eye is strongly indicated as a site for complement driven pathology. The most common cause of visual loss is age-related macular degeneration (AMD) where, in its more severe form (exudative or wet AMD), pathologic choridal neurovascular membranes develop under the retina. In the US, about 10% of the population aged 65-74 shows sign of macular degeneration and as many as 5% have visual impairment as a result to AMD. These numbers increase dramatically with age, but there are also genetic factors. Among the genes strongest associated with AMD are complement factor H, factor B and C3 and the C1 inhibitor (Bradley et al. Eye 2011, 25:683-693). Furthermore, several studies and clinical trials using various complement blocking molecules have proven beneficial, suggesting that a C5 blocking molecule could help these patient groups. However, the current treatments of advanced AMD aims at inhibition of vascular endothelial growth factor (VEGF) induced vascularization by intravitreal injections of e.g. Ranibizumab (a monoclonal antibody fragment) and Bevacizumab (monoclonal antibody). In animal models of uveitis, inflammation of the eye due to immune responses to ocular antigens, blocking antibodies against alternative pathway factor B (Manickam et al. J Biol Chem 2011, 286:8472-8480) as well as against C5 (Copland et al. Clin Exp Immunol 2009, 159:303-314), improved the disease state.
In transplantation of solid organs, there are two major mechanistic pathways leading to rejection or delayed/impaired function of the graft: 1) the immunologic barriers between donor and recipient with respect to blood group (ABO) and MHC classes as well as extent of pre-sensitization of the recipient against the donor. i.e. occurrence of donor specific antibodies (DSA) leading to acute antibody mediated rejection (AMR); and 2) the condition of the transplanted organ as well as the period of time it has been kept without constant blood perfusion. i.e. the degree of ischemic damage or ischemia reperfusion injury (IRI) of the graft. In both AMR and IRI, the complement system is attacking the organ recognized as foreign and, therefore, an entity that should be rejected. In AMR, the pre-existing anti-donor antibodies rapidly form immune complexes on the surface of the foreign organ leading to recognition by C1q and subsequent activation of the complement system via the classical pathway. This process, known as hyper-acute rejection happens within minutes and, therefore modern transplantation of mismatched organs includes elimination of DSA prior to transplantation by plasmapheresis or plasma exchange and intravenous IgG combined with different immunosuppressants. Novel treatments also include B-cell depletion via usage of the anti-CD20 antibody Rituximab (Genberg et al. Transplant 2008, 85:1745-1754). These protocols have vastly eliminated the occurrence of hyper-acute rejection but still, in highly sensitized patients, the incidence of acute AMR (weeks-months) is as high as 40% (Burns et al. Am J Transplant 2008, 6:2684-2694; Stegall et al. Am J Transplant 2011, early on-line publication). With respect to IRI, most evidence points at the terminal pathway with subsequent MAC formation and lysis as the main cause of tissue damage. Thus, a C5 blocking polypeptide would be protective against rejection regardless of the cause being AMR, IRI or, as often happens, a combination of both AMR and IRI. As expected, highly perfused organs, such as the liver (Qin et al. Cell Mol Immunol 2006, 3:333-340), the heart and the kidneys are particularly susceptible to complement mediated damage.
The central placement of the C5 protein; connecting the proximal and the terminal parts of the complement cascade, makes it an attractive target for pharmaceutical intervention. Since C5 is common to all pathways of complement activation, blocking of C5 will stop the progression of the cascade regardless of the stimuli and thereby prevent the deleterious properties of terminal complement activation while leaving the immunoprotective and immunoregulatory functions of the proximal complement cascade intact.
Antibodies targeted to human complement C5 are known from, e.g., WO 95/29697; WO 02/30985; and WO 2004/007553. Eculizumab (SOLIRIS) is a humanized monoclonal antibody directed against protein C5 and prevents cleavage of C5 into C5a and C5b. Eculizumab has been shown to be effective in treating PNH, a rare and sometimes life threatening disease of the blood characterized by intravascular hemolytic anemia, thrombophilia and bone marrow failure, and is approved for this indication. Eculizumab was also recently approved by the FDA for treatment of atypical hemolytic syndrome (aHUS), a rare but life threatening disease caused by loss of control of the alternative complement pathway leading to over-activation manifested as thrombotic microangiopathy (TMA) leading to constant risk of damage to vital organs such as kidney, heart and the brain. In aHUS, transplantation of the damaged organ only temporarily helps the patient as the liver continues to produce the mutated form of controlling protein (most often complement factor H or other proteins of the alternative pathway). A related disease with a transient acute pathophysiology is HUS caused by infection of Shiga toxin positive E. coli (STEC-HUS) and there are promising clinical data suggesting efficacy also for this condition (Lapeyraque et al, N Engl J Med 2011, 364:2561-2563). Finally, the C5 blocking antibody Eculizumab has proven efficacious in preventing AMR in recipients of highly mismatched kidneys (Stegall. M. D. et al. Am J Transplant 2011, 11:2405-2413).
Apart from full length antibodies, single-chain variable fragments (scFV), minibodies and aptamers targeting C5 are described in literature. These C5 inhibitors may bind to different sites (epitopes) on the C5 molecule and may have different modes of action. For example, whereas Eculizumab interacts with C5 at some distance of the convertase cleavage site, the minibody MUBODINA interacts with the cleavage site of C5. The C5 inhibitory protein Ornithodoros moubata Complement Inhibitor (OmCI, Nunn. M. A. et al. J Immunol 2005, 174:2084-2091) from soft tic Ornithodoros moubata has been hypothesized to bind to the distal end of the CUB-C5d-MG8 superdomain, which is close to the convertase cleavage site (Fredslund et al. Nat Immunol 2008, 9 (7):753-760). In contrast to the three proteins mentioned above inhibiting cleavage of C5, the monoclonal antibody TNX-558 binds to a C5a epitope present both on intact C5 and released C5a without inhibiting the cleavage of C5. (Fung et al. Clin Exp Immunol 2003, 133 (2):160-169).
Antibodies with their large, multidomain structure, 12 intra-chain and 4 inter-chain disulfide bridges and complex glycosylation patterns, have a number of intrinsic disadvantages related to their molecular structure. For example, the size of Eculizumab is about 148 kDa. The concentration of C5 in human blood is about 400 nM and in order to block C5 activity entirely, the concentration of the inhibitor must be at least equal or higher than that. Therefore, the standard life-long treatment regimen of PNH using SOLIRIS is intravenous infusions of 900 mg protein every second week, a treatment that mainly take place in the clinic leading to great inconvenience to the patient and cost to the society. SOLIRIS has also been reported to cause chest pain, fever, chills, itching, hives, flushing of the face, rash, dizziness, troubled breathing, or swelling of the face, tongue, and throat, although the reasons for these side effects are not clear. Furthermore, Eculizumab is not active in any tested animal model, including primates, making animal studies with the active drug impossible. As mentioned above, the current treatments of AMD are also antibody dependent and, thus, treatments based on injections or other routes of administration with molecules of lower molecular weight, are highly required.
In addition, antibody production is more difficult and more expensive than production of small proteins (Kenanova et al. Expert Opin Drug Deliv 2006, 3 (1):53-70). Other drawbacks generally related to antibodies are listed by Reilly et al. (Clin Pharmacokinet 1995, 28:126-142), such as cross-reactivity and non-specific binding to normal tissues, increased metabolism of injected antibodies and formation of human anti-human antibodies (HAMA) causing decreased or loss of the therapeutic effect.
Thus, continued provision of agents with comparable C5 blocking activity remains a matter of substantial interest within the field. In particular, there is a continued need for molecules that prevent the terminal complement cascade as well as the formation of the pro-inflammatory molecule C5a. Of great interest is also a provision of uses of such molecules in the treatment of disease.